let the radio0/1 to decide what channel and power should be used, when you have country-code correctly set you should have values within country norm/required limits …
the metrics for signal/signal-to-noise and so on, that depends on how many devices are around, how many devices are trying to connect and if you are broadcasting around or not. use some speed testers.
sure you can be deflected by more powerfull signals around, but 5G should be fine within your flat, 2,4G one might get issues, but there should not be much users on that frequency.
Now if you can have more power you can increase dB output or short the range (so you are not broadcasting too far.) 5G is fine when you are in same room, to get max power you should be within 2m from router, in next room it might be better to switch to 2,4G one.
I would not fiddle with channels (use auto if you can), because you pick one during morning (totally free) and during night it is opposite (totally full) and if you do not have auto you always pick that one …
you can have some script to check the occupation of used channel and wifiup/wifidown the AP in that case so autochannel hit in and pick correct one.
If you have strong penetration by one ISP providing some fixed setup (like UPC …where you have guest wifi autoconfigured and WPS active and own wifi and own guest …voila you have lot of APs broadcasting around from each flat :((( )
in my case i have 1,2G speed on interlwirelesspro wifi card (that one with dual interface). even if the AP shows only like 350M …due MIMO there was lik 2-3x380-650M + 1-2x38-54M +1-2x6-12M, based on distance those changes so when far from router there was only 1-2 mimo active and then signal drops and speed get slower. FYI and my metrics in luci or statistics seems like garbage, like i have no signal or speed (max there is 200M, but i know it can do more == so more informative values are aside each client in dhcp overview for each wlan …there you will see speed per each mimo channel)
When you have another device active , it atract the signal in same way, so it can steal some of the “mimo” channels and your speed seems slow.
If you have guest wifi on one of your wlans you share the resources …, so you have to consider all aspects of your lan/wlan.